SARS-CoV-2 evolved an incubation time more like seasonal coronaviruses

SARS-CoV-2 evolved an incubation time more like seasonal coronaviruses

The incubation period for COVID-19 – the time between when SARS-CoV-2 first infects a person and when the resulting COVID-19 symptoms first appear – is is gradually shortened as the pandemic has spread and the virus has mutated. It depends a new meta-analysis published this week in JAMA Network Open by researchers in Beijing, who collected data on more than 8,000 patients from 142 COVID-19 studies.

When the original version of the new virus emerged from Wuhan, China, the average incubation period was 6.65 days, according to pooled data from 119 studies. But then the incubation period got shorter as the variants evolved. The alpha variant had an average incubation of 5 days, according to one study; beta, 4.5 days, according to another; delta had an average of 4.41 days, based on pooled data from six studies; and now with omicron, the incubation period has been reduced to 3.42 days, according to data from five studies.

The current shortened incubation period now puts SARS-CoV-2 more in line with common respiratory viruses, including the four human coronaviruses that circulate seasonally and cause mild infections similar to the common cold. Their incubation period is 3.2 days. Rhinovirus, the most common cause of the common cold, has an average incubation period of 1.4 days. For influenza, it can range from 1.43 to 1.64 days, and parainfluenza has an average of 2.6 days.

better or worse

In terms of disease severity, the importance of a shorter incubation period is not entirely clear, which has been shown by studies included in the meta-analysis that broke out specific groups of people, including older adults, children, and people who have developed severe COVID -19.

For example, the pooled data from eight studies that estimated the incubation period only in people over 60 – people who are relatively high risk of severe COVID-19 – found that they tended to have slightly longer incubation periods, averaging 7.43 days. This matches previous data, which found that older adults also had longer incubation periods during the initial outbreak of the SARS virus in the early 2000s. Researchers speculate then and now that the period of longer incubation in older people reflects slower immune responses to the virus.

But the children, who are relatively down risk of severe COVID-19, also tended to have relatively longer incubation periods with SARS-CoV-2. Their average incubation period was 8.82 days, according to pooled data from eight studies. The authors believe this may be because symptoms in children are so mild that detection of COVID-19 symptoms may be delayed.

To further complicate the picture, pooled data from six studies looking specifically at incubation periods in people who developed severe disease and in those who developed non-severe disease. In this comparison, patients with severe COVID-19 tended to have shorter incubation periods (6.69 days) than those with non-severe cases (6.99 days). The authors of the meta-analysis believe this could be related to the fact that people with severe disease start out with more cells initially infected with the virus than those with only mild disease.

“Great importance”

Overall, the complex relationship between incubation time and severity of COVID-19 highlights that the disease depends on various factors, especially virus-related factors (i.e. virus virulence and infectious dose) and factors specific to the human host (i.e. immune system function and prior immunity to infection or vaccination). Omicron, the newest variant and the one with the shortest incubation period to date, is considered to cause relatively mild disease. But it also appeared after widespread vaccination and previous infections, which usually protect against serious diseases.

Yet, despite the complexity, the incubation period is “one of the most important epidemiological parameters of infectious diseases,” the authors write. “Knowledge of the incubation period of the disease is of great importance for case definition, management of emerging threats, estimation of follow-up duration for contact tracing and secondary case detection, and the establishment of public health programs aimed at reducing local transmission. such as social distancing, isolation, face mask mandates and quarantine. This is particularly critical for SARS-CoV-2, which has been shown to be highly efficient in pre-symptomatic transmission.

The meta-analysis has several limitations. Like all meta-analyses, it harvested data from various datasets from studies conducted in many different countries, creating the potential for confounding variables. Some of the data was also based on people’s recall of exposure dates. Finally, the majority of studies included in the review were for the initial version of SARS-CoV-2. Thus, estimates of incubation periods for newer variants were based on fewer data. However, the general conclusion of the meta-analysis was echoed by others who found that the incubation period has shortened during the pandemic and, in the age of the omicron, is now l order three to four days.

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